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Introduction and Baseline Mission Requirements

INTRODUCTION

The human exploration of Mars is a daunting undertaking. Safely transporting astronauts to and from Mars will require advances in many areas to develop spacecraft that are up to the challenge. Propulsion systems are one such area. Advanced nuclear propulsion systems (alone or in combination with chemical propulsion systems) have the potential to substantially reduce trip time compared to fully nonnuclear approaches. Shorter trip time reduces risks associated with space radiation, zero gravity, launch and orbital assembly requirements, and many other aspects of long-duration space missions.

This report assesses the primary technical and programmatic challenges, merits, and risks for developing a nuclear thermal propulsion (NTP) system or a nuclear electric propulsion (NEP) system augmented with a chemical propulsion system for the human exploration of Mars. The report also includes NEP and NTP development roadmaps with key milestones.

Many NASA studies have considered the use of NTP or NEP to facilitate the human exploration of Mars.^1,2,3 Mission scenarios associated with nuclear, solar, and chemical propulsion systems and various mission parameters are shown in Table 1.1. Launch assumptions varied with the launch systems in use or under development at the time of each study. Because crewed Mars missions are significantly more challenging in terms of launch mass and trip time than all prior space missions, in-space propulsion is a critical technology. This is evident by the wide range of propulsion systems that have been considered over multiple mission studies.

Based on the relative orbits of Mars and Earth, the distance between Earth and Mars ranges from 55 to 400 million km over a synodic period of approximately 26 months. Launch (or Earth departure) requirements vary significantly over this cycle.

Each 26-month cycle is not the same. Propulsion system performance requirements, in terms of the total velocity increment (∆V) of a round-trip Mars mission, vary from one launch opportunity to the next. The ∆V for a particular mission also depends on other mission constraints, particularly the stay time at Mars and the desired trip time.

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¹ D. Portree, “Humans to Mars: Fifty Years of Mission Planning, 1950-2000,” Monographs in Aerospace History #21, NASA SP-2001-4521, 2001, https://history.nasa.gov/monograph21/humans_to_Mars.htm.

² Explore Mars, Inc., The Humans to Mars Report, 2000, https://www.exploremars.org/wp-content/uploads/2020/08/H2MR_2020_Web_v1.pdf.

³ NASA, Human Exploration of Mars Design Reference Architecture 5.0, NASA SP-2009-566, 2009, https://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf.

TABLE 1.1 Mission Scenarios for Crewed Mars Missions

There are two classes of crewed missions to Mars: conjunction class and opposition class. Conjunction-class missions have the lowest ∆V requirements. For crewed conjunction-class missions, trip times are typically 180 to 210 days each way, stay times on Mars are typically 500 days or more, and total mission time is around 900 days.⁴ These are the “long stay” missions in Table 1.1.

In contrast, one leg of opposition-class missions occurs when the orbital alignment of Earth and Mars is less favorable, but they allow for short stays on the surface of Mars (“short stay” missions in Table 1.1). These missions have higher ∆V requirements and require more propellant, which increases the mass of the Mars vehicle and the number of launch vehicles necessary to lift the required mass to its assembly orbit. Opposition-class missions are characterized by much shorter stay time on Mars (30 to 90 days) and a shorter total mission time (400 to 750 days). An additional complexity of opposition-class missions is that the long leg of the mission typically passes inside Earth’s orbit, generally as close to the Sun as the orbit of Venus, to mitigate the adverse planetary alignment of that leg of the mission. This results in both thermal and radiation challenges for a crewed Mars mission. Representative trajectories for each of the crewed mission scenarios are shown in Figure 1.1.⁵

BASELINE MISSION TO MARS: CREWED OPPOSITION-CLASS MISSIONS

The baseline mission specified by NASA for this report is an opposition-class crewed mission to Mars launched in 2039. This mission would be preceded by cargo missions beginning in 2033 to pre-place surface infrastructure and consumables for the crew. The propulsion system needed for this mission would also be sufficient for conjunction-class missions. The baseline mission has the following parameters:

• Crew mission launch in 2039 opportunity;
• Total crew trip time ≤750 days;⁶
• Split mission with separate crew and cargo vehicles,
  • Same propulsion systems used on all vehicles,
  • Cargo vehicles arrive at Mars prior to first crew departure from Earth;

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⁴ NASA, Human Exploration of Mars: Design Reference Architecture 5.0 Addendum, NASA-SP-2009-566-ADD, 2009, https://www.nasa.gov/pdf/373667main_NASA-SP-2009-566-ADD.pdf.

⁵ Exact trajectories would depend on many parameters such as launch date and the nature of the propulsion system.

⁶ Some hardware will have a total mission time of perhaps 4 years, assuming 2 years in an assembly orbit and round-trip flight time of 2 years.

• Stay time on the Mars surface of 30 days;
• Crew of four, two of whom land on Mars; and
• Vehicle systems, cargo, and propellant launched by multiple launch vehicles to an assembly orbit, which would be either in low Earth orbit or cislunar space.

In order to meet the requirement for total trip time with an NEP system, Earth departure and Mars capture and departure would be augmented by an additional in-space liquid methane and liquid oxygen chemical propulsion system. The NEP system provides acceleration and deceleration in interplanetary space. In contrast, the NTP system provides propulsion for all transit maneuvers. The mission segments and the propulsion system used for each phase of flight are described in Table 1.2.

As Earth and Mars revolve about the Sun, the most efficient trajectories vary, resulting in varying levels of propulsive requirements (∆V) over a 15- to 17-year period (see Figure 1.2).

A factor in mission assessment for repeated trips to Mars is the ability of propulsion systems to meet mission ∆V requirements over a series of consecutive launch opportunities without large variability in overall mission parameters, such as propellant mass, which could drive very different launch requirements for different opportunities. This variability is reduced by propulsion systems with high specific impulse (I_sp).⁷ Previous studies have shown the impact of NTP for an opposition-class mission in different launch opportunities, although not for the current years of interest. An example of the change in vehicle (propellant) mass with launch date is shown in Figure 1.3 for an advanced chemical system with an I_sp of 480 s and an NTP system with an I_sp of 825 s. The mass variation with launch opportunity for the higher I_sp system is about one half of the variation of the chemical system. Similar benefits would likely be achieved with an NEP system with an I_sp of 2,000 s paired with a conventional chemical system. This is particularly important because some launch opportunities are not feasible using purely a chemical system. Flexibility to launch date is a major architectural advantage of the use of nuclear propulsion.

PROPULSION SYSTEM REQUIREMENTS

Although NEP and NTP systems both use nuclear power, they convert this power into thrust in different ways based on different technologies (as will be discussed in Chapters 2 and 3). The performance of rocket propulsion systems is defined by multiple parameters that define how much propellant they use and how much acceleration they can generate. In the case of chemical rockets or NTP systems, the two primary parameters are the I_sp and thrust. For NEP systems, I_sp is important to determine propellant requirements, but thrust and acceleration are defined by multiple parameters: power, thrust efficiency, and specific mass. Thrust efficiency defines how much electric power is converted into thrust power, and the specific mass is defined as the mass of the entire NEP system divided by the electrical power available for the thrusters. NEP systems have a higher I_sp than NTP systems, but they have very low thrust. The megawatt electric (MWe)-class NEP systems proposed to execute the baseline

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⁷ I_sp is the thrust of a rocket (or electric thruster) divided by the weight flow rate of the propellant. The unit for I_sp is seconds.

mission therefore require chemical rockets (which have an I_sp that is much lower than either an NTP system or an NEP system) to meet the desired trip time.

NTP and NEP system performance requirements to execute the baseline mission are a topic of ongoing study by NASA. Table 1.3 summarizes the committee’s estimate of those requirements for NTP and NEP systems based on information from multiple sources.⁸

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⁸ To meet the trip time specified for the baseline mission, the requirements for a pure NEP system (without an auxiliary chemical propulsion system), would include a much higher power level and a much higher I_sp than those specified in Table 1.3. M. McGuire et al., “Use of High-Power Brayton Nuclear Electric Propulsion (NEP) for a 2033 Mars Round-Trip Mission,” AIP Conference Proceedings 813, 222, 2006. doi:10.1063/1.2169198.

CARGO MISSIONS

Conjunction-class missions have the lowest possible ∆V requirements because they use minimum energy, or Hohmann-like, trajectories. These trajectories are traditionally cited for cargo missions in which mass efficiency rather than trip time is a priority. Cargo missions also benefit from the higher I_sp of NEP and NTP systems. To ensure delivery of the requisite payloads to Mars before launch of crew, multiple cargo flights are planned as an integral aspect of this enterprise. As discussed in Chapters 2 and 3, using the crew vehicle propulsion system on one or more of the precursor cargo vehicles provides significant risk reduction and valuable flight information about propulsion system reliability, safety, and performance.

SUMMARY

NASA is presently considering multiple forms of propulsion, including NTP and NEP, in its mission architecture analyses. Opposition-class missions, while reducing crew duration on Mars and total mission time, markedly increase mission ∆V requirements. This mission class introduces a higher sensitivity in propulsion system requirements from one launch opportunity to another, which could be achieved by either an NTP or NEP system. Successful development of an NTP or NEP/chemical system at relevant scale and performance would allow NASA to develop a robust architecture with flexibility across multiple mission opportunities.

This report provides a technology assessment of the NTP and NEP development challenges that must be overcome to execute the baseline Mars mission. It is not intended to provide—nor did the committee’s statement of task allow—a comprehensive assessment of all aspects or trade studies associated with how a human Mars exploration mission should be organized, funded, or executed.